EFFECT OF THE INTRASEASONAL WIND FLUCTUATIONS IN THE WEST AFRICAN MONSOON ON
AIR-SEA FLUXES.
Semyon A. Grodsky, and James A. Carton
Department of Meteorology, University of Maryland, College Park, MD 20742, USA
E-mail: senya@ocean2.umd.edu
Abstract. The winds of the tropical Atlantic are known to have strong fluctuations on
periods of a few days. These African Waves result from instabilities in the tropical winds
crossing Northwest Africa and in some cases develop into the tropical storms and
hurricanes of the western Atlantic [Yanai and Mukarami, 1970; Viltard et al., 1997;
Diedhiou et al., 1998]. Here we use new surface wind observations made available by the
QuickSCAT satellite and rainfall estimates from the Tropical Rainfall Measuring Mission
(TRMM) to reexamine fluctuations in the synoptic meteorology of tropical West Africa and
the Atlantic. We find the additional presence in surface winds and rainfall of strong quasitwo week disturbances in northern spring and summer. These disturbances appear to
result from a cycle of continental heating, convection, and cooling that gives rise to
fluctuations in the monsoon system.
1.
INTRODUCTION
Intensive study of synoptic-scale disturbances over central and West Africa and the tropical Atlantic
has revealed the existence of two types of westward propagating wave-like fluctuations. The first are the
African Waves with zonal wavelengths of 2,500-3,000 km, periods of 3-5 days, and westward speeds of 9-10
m/s and which are most evident in the meridional component of winds [Carlson, 1969; Viltard et al., 1997].
The second type has longer 6-9 day periods, correspondingly longer 6,000 km zonal wavelengths, and
higher, 11 m/s, westward speeds [Yanai and Mukarami, 1970]. These latter wave-like fluctuations have their
strongest expressions in zonal surface wind and cloud cover in the latitude band 10 0-200N [Oubuih et al.,
1999]. Recently Grodsky and Carton [2001] have used daily data of the year 2000 provided by the SeaWinds
scatterometer aboard QuickSCAT satellite [Spencer et al., 2000] and Tropical rainfall estimates from the
Tropical Rainfall Measuring Mission satellite microwave sensor suite [Kummerow et al., 1998] to report on a
third class of intraseasonal disturbances in the winds of the eastern tropical Atlantic. They occur closer to the
equator, develop inside and close to the ITCZ area during April - June, and seem do not propagate
westward. The spatial extent of the wind oscillations changes with season and is closely related to the
meridional shift of the ITCZ. Janicot and Sultan [2001] have also observed a quasi-periodic signal of about
15 days in the rainfall and wind fields over West Africa based on a longer 1968-1990 data set. The presence
of the quasi-biweekly fluctuations in the reanalysis zonal winds over the eastern tropical Atlantic and
Northwest Africa have been also noted by Viltard et al. [1997]. Moreover, the existence of bi-weekly
oscillations in the tropical wind system is not unique to Northwest Africa as very similar oscillation was found
by Krishnamurti and Bhalme [1976] in the Indian monsoon. These studies provide further evidence that
quasi- biweekly period is an important time scale of oscillation in the monsoon-like system that develops due
to its own inherent dynamics.
2
RESULTS.
It is evident from Figure 1 that the pattern of winds and rainfall can undergo rather rapid changes.
The 12-day period beginning May 17 illustrates a reversal of the normal trade winds between 2 0N and 80N
and east of 250W. This change brings moist maritime air eastward onto the areas surrounding the Gulf of
Guinea resulting in intermittent heavy rainfall at least a month prior to the annual rainy season. The eastward
shift of convection causes a corresponding reduction in rainfall in the western side of the basin (May 22
panel). During the succeeding week (late May- first days of June) the winds return to their normal
configuration, while convection shifts westward onto the ocean and towards eastern Brazil (not shown in
Figure 1). The trade winds reversal seems to be the result of interactions between the continental hydrologic
cycle, summertime heating, and the tropical trade winds.
The sequence of wind, rainfall, and land heating cycles leading to development of the biweekly
oscillation is further illustrated by the scheme presented in Figure 2. During normal (eastward) trade winds
the convection is shifted seaward and sunshine over land increases the land temperature that lowers
atmospheric pressure (upper panel). Decline in air pressure over land reverses wind direction that shifts
convection towards the land bringing in maritime moisture, rainfall and cloudiness (lower panel). This
decreases surface temperature over the land and restores (increases) atmospheric pressure, and the trade
winds switch to normal direction shown in upper panel. Grodsky and Carton [2001] have shown using the
NCEP/NCAR reanalysis of Kalnay et al. [1996] that variations of the land temperature, sea-land pressure
difference, and rainfall over the land occur in close correspondence with wind oscillations within the ITCZ
west off the African coast.
The wind oscillation
changes wind speed and
direction that, in turn, shifts
the
area
of
intense
convection in zonal direction.
Here we illustrate the impact
of these processes on airsea fluxes using daily
NCEP/NCAR
reanalysis
data. The air-sea fluxes will
be estimated west off African
coast over the area bounded
by 40N-60N, 200W-100W. The
largest impact is expected
on the Short Wave Radiation
(SWR)
as
it
depends
strongly on cloud coverage.
Thus, the SWR must be
maximal during the normal
easterly trade winds when
the
convection
shifts
seaward towards the west.
As trade winds reverse and
convection shifts towards the
African coast, the SWR
becomes smaller due to
shadowing effect of clouds.
The Latent Heat Flux (LHFL)
is expected to change in a
more complicated way as it
depends on both, wind
speed and relative humidity.
Figure 1. Two-day average surface winds and rainfall. Rates exceeding
If we disregard the effect of
0.5 mm/hr are shaded. This pattern has switched back by 30 May.
relative humidity, the LHFL
simply follows the changes
in wind increasing as wind
speed rises regardless of
wind direction.
Figure 3 presents
the components of the airsea surface net heat flux
along with 3-day average
zonal wind. The flux data are
3 – 60 day band passed and
shown during April – June
when a succession of 5 biweekly wind oscillations is
evident. Main contribution to
net heat flux change comes
from variation in SWR and
LHFL, as was supposed
earlier. Both components,
LHFL and SWR, vary about
±20 W/m2. Their average
values are <LHFL>=78 W/m2
Figure 2. The scheme of trade winds direction and convective clouds during
and <SWR>=205 W/m 2.
the normal easterly winds (upper panel) and trade winds reversal (bottom
Intraseasonal variation of the
panel).
LHFL displays correlation
with zonal wind speed
modulus (Figure 4, upper
panel). This correlation is
obvious during May when
the
strongest
oscillation
occurred. The correlation
isn’t unique because of
impact of air humidity.
Short wave radiation
is expected to vary with
phase opposite to that of the
zonal wind. This regularity is
observed in Figure 4 (bottom
panel) beginning 16-th of
May. Before this date, the
relationship between SWR
and zonal wind is less
definite. Possible reason for
that may relate to insufficient
accuracy of the cloud
parameterization used by the
reanalysis model.
3
RESUME
This paper presents
observational evidence of
the existence of quasi biweekly oscillations in the
tropical wind system over the
eastern Atlantic. Oscillatory
regime related to feedbacks
within the hydrologic cycle is
not unique to Northwest
Africa. Lau and Bua [1998],
for
example,
present
modeling evidence of a
similar
phenomenon
occurring in the region of
East Asia/Indochina. These
studies
provide
further
Figure 3. Three day mean zonal wind, U, and 3 – 60 day band passed
evidence
of
strong
latent heat flux, LHFL, sensible heat flux, SHFL, long wave radiation, LWR,
interactions
between
and short wave radiation, SWR. All values are averaged over 4 0N – 60N,
continental and maritime
200W – 100W.
atmospheric processes and
the hydrologic cycle.
It is much probable that zonal wind oscillations in the tropics might force an oceanic response. The
biweekly oscillations strongly modify winds in vicinity of the ITCZ that may influence the wind-driven
circulation of the tropical ocean. Note also that during the trade winds reversal an upwelling favorite wind
westerly blows along the northern coast of the Gulf of Guinea. All these issues are awaiting future studies.
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Figure 4. (Upper panel). Latent heat flux, LHFL, (3 – 60 day band passed, bold line) and 3-day mean
zonal wind modulus, |U|, (thin line).
(Bottom panel). Short wave radiation, SWR, (3 – 60 day band passed, bold line) and 3 –day mean
zonal wind, U, (thin line). All values are averaged over 40N – 60N, 200W – 100W.